Woodworkers and carriage makers long have inserted wood plugs cut across the grain of wood in countersunk screw holes, or similar holes, to hide the screw or otherwise fill or conceal the hole. Plugs cut from across the wood grain are more desirable than plugs cut from end grain, such as dowels, because cross grain plugs more closely match the grain and texture of the wood being plugged, and when stained or otherwise finished, provide a nearly invisible plug.
Two types of plugs have been long known, cylindrical plugs with straight sides and tapered plugs with sides that narrow slightly. Various chamfers have been used on both cylindrical and tapered plugs. For example, U.S. Pat. Nos. 117,786, 473,231, 475,560, 476,312, 2,748,817, 3,130,763, and 4,595,321 describe plug cutters which will cut cylindrical plugs. U.S. Pat. Nos. 2,027,139 and 4,295,763 describe plug cutters which will produce tapered plugs. However, neither plug type has been entirely satisfactory. Cylindrical plugs must necessarily be made slightly smaller than the hole being plugged so that the plug can be inserted easily. This size difference frequently leaves a small, noticeable gap between the plug and the hole side, particularly because tolerances are difficult to control in forming both the cylindrical plugs and the holes which are to receive the plugs. For instance, this gap is magnified if the hole is not perfectly round. Furthermore, cylindrical plug cutters tend to tear and burn rather than cut the fibers on the side of the plug. This tearing and burning produces a roughened plug, thereby causing a poor fit, difficult insertion, and voids which can be exposed when the top of the plug is finished level with the workpiece.
In addition, inserting a cylindrical plug often involves tilting the plug, thereby crushing the fibers on one side of the hole and producing a noticeable gap. Finally, cylindrical plugs tend to rotate during the final step of insertion, when they are typically tapped into place, thereby causing the grain of the plug to be misaligned with the grain of wood of the workpiece into which the plug is being inserted. This misalignment causes the plug to be much more noticeable and is therefore undesirable.
Tapered plugs also have their drawbacks. Although tapering the plug helps to reduce the gap between the plug and the side of the hole, tapered plugs contact the workpiece only at a thin ring adjacent to the finished surface, which allows the plug to rock within the hole and does not provide an adequate gluing surface. Consequentially, tapered plugs are not as secure as cylindrical plugs and may work loose or cause an uneven finished surface. Indeed, surface sanding of the plug and adjacent area after the plug has been secured can destroy the ring of contacting surface, causing the tapered plug to be loosened or revealing a gap between the plug and workpiece.
U.S. Pat. No. 4,295,763 (Cunniff), describes a device that cuts a large diameter plug with two different tapers for plugging holes bored in exterior walls for the insertion of insulation into the wall cavities. However, the Cunniff device does not solve the above-described problems associated with prior art plugs because plugs cut with the Cunniff device are not appropriate for conventional woodworking applications and are inserted with the steeper taper first (which thus functions as a chamfer). As a result, the second taper contacts the hole side and there is minimum contact between the plug itself and the side of the hole, reducing the gluing surface and increasing the likelihood that the plug will rock.